Cellular phones generally include a main body on which operating buttons are provided, and with a movable body which either rotates or slides relatively to the main body. A circuit board provided on the main body and a circuit board provided on the movable body are connected together by means of a flexible signal line (hereinafter, referred to as a flexible cable). A cable formed by housing a thin metal wire inside a flexible resin tube, or a flexible wiring substrate is employed as the flexible cable.
For example, in the case of a folding type of cellular phone, the movable body is pivotably joined by means of a hinge portion to the main body, and the flexible cable is laid inside the device so as to pass adjacent to the center of rotation formed by the hinge portion of the movable body (see, for example, Patent documents 1 and 2).
In recent years, there have been demands for increases in both speed and transmission capacity in signal transmissions inside electronic devices such as gaming devices, electronic dictionaries, PDA (personal digital assistants), personal computers, and measuring instruments such as testers. In consideration of these demands, it is becoming common for optical communication technology, which uses a photoelectrical composite substrate which is formed by stacking optical waveguide forming components (these may be referred to below simply as optical waveguides) and sheet-shaped optical waveguides on a circuit board so as to create an integrated body in order to be applied to a signal transmissions within a device.
In addition to conventional quartz-based optical waveguides, polymer-based optical waveguides which are easy to manufacture (i.e., to pattern) and have widespread applicability are used as the optical waveguide. In recent years, development of the latter optical waveguides has been carried out vigorously. It is normal for these optical waveguides to be treated as a single substrate forming component either independently, or by being stacked on a circuit substrate like the aforementioned photoelectrical composite substrate, and to be formed as a rigid material.
Moreover, among these rigid optical waveguides, in order to reduce transmission loss in transmission light (here, this refers particularly to radiation loss), optical waveguides are known where air contacting with a core portion via holes and trenches and the like formed in cladding portions functions as over-cladding, and provides an improved light confinement effect (see, for example, Patent documents 1, 2, and 3).    [Patent document 1] Japanese Patent Application, First Publication No. 2005-091469    [Patent document 2] Japanese Patent Application, First Publication No. 2005-070324    [Patent document 3] Japanese Patent Application, First Publication No. 2003-207661